lucy24 - 3:50 am on Apr 16, 2011 (gmt 0)
If you are only just setting up the site, a sitemap-- assuming you mean the sitemap.xml file, not a "sitemap" page visible to humans-- may well be worse than useless. (Voice Of Experience.) When you move things around and submit a fresh sitemap, google doesn't purge the information from the old one. They keep right on trying to crawl those nonexistent pages. So you get a bunch of spurious "crawl errors" that you have to waste time investigating.
If everything on your site can be reached by following links from the front page, a sitemap shouldn't be necessary at all. Unless you have a large Russian audience. Yandex simply adores my sitemap; it's their first stop on every visit.